The present invention relates to an anisotropically electroconductive resin film having electroconductivity in the thickness direction alone via the electroconductive particles dispersed in the film, film-like material, a process for electrically connecting circuits and a connected structure using said film, and an electronic part testing process using the connected structure.
Miniaturization of electronic parts has entailed higher density and higher fineness of the circuits used therein. As the conventional solder or rubber connectors can hardly meet the connecting specifications of these fine circuits, anisotropically electroconductive adhesives or connecting means made of a film are popularly used recently. In these methods, a layer of electrical connecting means made of an insulating resin containing a specified amount of an electroconductive material is disposed between the opposing circuits and pressed, with heating if necessary, to set up electrical connection between the upper and lower circuits as well as electrical insulation between the adjoining circuits. It is also common practice to use insulating resin as an adhesive for making electrical connection between the opposing circuits and fixing thereof.
Prior art literature relating to anisotropically electroconductive resin films having electroconductivity in the thickness direction alone includes Japanese Patent Unexamined Publication (JP-A) 51-21192 which discloses a process for producing said type of resin film according to which a mixture of electroconductive particles and a nonconductive base resin which keeps said particles uncontacted with each other is molded into a sheet having a thickness substantially equal to the size of the particles to provide a sheet-like product having electroconductivity in the thickness direction alone, and Japanese Patent Post-Exam Publication (JP-B) 59-31190 which discloses a sheet-like one-piece electrical connector comprising a homogeneous mixture of electroconductive particles (0.05-20 vol %) and a flexible insulating binder. In these prior art disclosures, a molding of a resin film material having a desired thickness is obtained by rolling a mixture of a resin and electroconductive particles homogeneously dispersed therein, or by casting a liquid resin having electroconductive particles homogeneously dispersed therein to a desired thickness by a bar coater or other suitable means and then drying or curing the casting.
For obtaining a film in which the concentration of electroconductive particles differs from one direction to the other, that is, the film thickness direction, there are known a method in which electroconductive particles are contained in a porous film and then said film and electroconductive particles are bonded and fixed (Japanese Utility Model Unexampled Publication (JUM-A) 61-196413), and a method in which electroconductive particles are embedded in an adhesive film (JP-A-2-117980 and JP-A-5-67480).
When obtaining circuit connection by disposing an anisotropically electroconductive film having electroconductivity in the thickness direction alone between the circuits and pressing them into a contacted state, it is effective to have the individual electroconductive particles exposed on both sides of the film for reducing connection resistance. As means for exposing the electroconductive particles on both sides of the film, besides those disclosed in the abovementioned patents, methods are known in which the film is rolled (JP-A-61-23507 and JP-A-61-188818), or rolling and sputter etching are used in combination (JP-A-61-200616). Methods are also usable in which electroconductive particles are contained in a porous film and said film and electroconductive particles are bonded and fixed (JP-A-5-74512), or electroconductive particles are held between a pair of flat plates and then a liquid resin is filled between said plates and molded into a film (JP-A-2-239578). In other known methods, the surface layers of film-forming resin on both sides of the film are removed by dissolution or decomposition with a solvent, or by physical means such as sputter etching, plasma etching or excimer laser irradiation.
In these anisotropic conductive resin films having electroconductivity in the thickness direction alone, in order to obtain higher resolution by increasing the electroconductive points per unit area, it is necessary to increase the content of electroconductive particles in the film. In the functional test before mounting the electronic parts such as liquid crystal displays (LCD), tape automated bonding (TAB) integrated circuits, bare chip integrated circuits and the like, connection of fine electrodes has been made by use of a pin probe, a device by which a needle-like electrode is pressed into contact with the corresponding electrode of the electronic part to be tested.
Connection mechanisms such as shown in the following prior art literature have also been proposed. In the connection mechanism disclosed in JUM-A-53-156569, JP-A-54-67672, etc., an anisotropically electroconductive rubber sheet is sandwiched between the electrode section of an electronic part to be tested and an testing substrate for making electrical connection. Said anisotropically electroconductive rubber sheet comprises laminations of electroconductive rubber and insulating rubber, in which electroconductive particles are uniformly dispersed or electroconductive short fibers are contained with orientation in the thickness direction of the insulating rubber sheet. In the connection system disclosed in JP-A-3-183974, electroconductive particles are localized at the positions corresponding to the electrodes for the purpose of improving fineness of said anisotropically electroconductive rubber sheet. According to the electrical connecting method proposed in JP-A-59-155769 and JUM-A-59-163967, the conductive pattern formed on a flexible film is directly contacted with the electrode section of the electronic part to be tested. In the connection system disclosed in JP-A-61-2338, JP-A-1-128381, JP-B-2-44747, JP-B-3-22367, etc., an electroconductive protuberance is provided on the conductive pattern and this protuberance is directly contacted with the electrode section of the electronic part. Further, JUM-A-5-18031 proposes a connection mechanism in which an anisotropically electroconductive film having a protuberant electrode extending through an insulating polymer film is disposed between the part to be tested and a testing printed substrate. The prior art devices such as mentioned above, however, have their own problems. For instance, according to the film molding method comprising casting a homogeneous mixture of a liquid resin and electroconductive particles, when it is tried to increase the mixing ratio of the electroconductive particles in correspondence to high fineness of electrode, the viscosity of the liquid resin having the electroconductive particles dispersed therein elevates to impair fluidity of the liquid resin, making it difficult to perform constant-thickness casting by a bar coater or such means. This necessitates a decrease of the mixing ratio of electroconductive particles. Further, in the case of the film having electroconductive particles uniformly dispersed therein, when the mixing ratio of electroconductive particles is increased in conformity to high-fineness circuits, there is also inevitably increased the amount of those electroconductive particles which are not held between the connected circuits and make no contribution to electrical conduction, giving rise to the problem of elevated production cost. Also, in the method in which electroconductive particles are contained in a porous film and then said film and electroconductive particles are bonded and fixed, it is impractical in terms of productivity and cost to form a large number of minute holes in the film. In the method in which the electroconductive particles are embedded in an adhesive film, the adhesive won't be wetted sufficiently on the electroconductive particle surfaces unless the viscosity of the adhesive is sufficiently low to preserve the desired liquid state, and consequently adhesiveness of the filmy adhesive to the electroconductive particles is lowered, making the electroconductive particles liable to fall off the film. In case of using a liquid filmy resin, it is unavoidable that the adhesive stick to the support in the step of transferring the electroconductive particles held on a support to the liquid adhesive surface, making it difficult to mold a film. When the content of electroconductive particles is increased, it becomes difficult to rightly fill the adhesive in the spaces between the electroconductive particles, so that it is required to strictly regulate the relation between the filled amount of electroconductive particles and the thickness of filmy adhesive before filling of electroconductive particles. It should be further noted that when the thickness of filmy adhesive is large, sticking of the adhesive to the support is unavoidable and when the thickness is small, the desired film strength may not be obtained or the particles may fall off the film.
In the conventional method of exposing electroconductive particles on both sides of the film by means of rolling, it is difficult to uniformly reduce the film thickness to the order of several ten microns by reducing the size of electroconductive particles in conformity to the high-fineness electrodes, and variation in size of electroconductive particles causes corresponding scatter of film thickness.
In the method of forming a film by casting a homogeneous mixture of a liquid resin and electroconductive particles, when the casting thickness is enlarged for increasing the fill of particles in the lower layer of the resin film molding by sedimentation of electroconductive particles, there is inevitably constituted a multilayer structure of electroconductive particles, and as a result, the particles which take no part in making the film electroconductive in the thickness direction are increased. Also, in this method, the resin layer which must be removed later is thick, making it difficult to uniformly expose the particles on the film surface.
In the film forming method in which a liquid resin is filled between a pair of flat plates with electroconductive particles held therebetween, the viscosity of the liquid resin must be extremely low for filling the resin in a small gap of plates, and variation in size of electroconductive particles may cause flowing out of electroconductive particles. In case of exposing electroconductive particles on the film surface by dissolving away the resin on the particle surfaces with a solvent, the solution of the dissolved film-forming resin adheres to the film surface and, when dried, forms a thin film on the particle surface to impair electrical connection. Also, since the thickness of film-forming resin must be strictly controlled by treating time or treating temperature, scatter of thickness from lot to lot or scatter of partial thickness of the film tends to occur.
In the method in which the film-forming resin is decomposed with a solvent, the film-forming resins decomposable with a solvent and the solvents usable therefor are restricted, and care is required in treatment when using an acidic or alkaline solvent. In this case, too, the thickness of film-forming resin needs to be strictly controlled by treating time or treating temperature, so that there tends to arise scatter of thickness from lot to lot or scatter of partial thickness of the film.
In the method comprising physical removal of resin by means of sputter etching, plasma etching or excimer laser irradiation, the apparatus is expensive, the treating time is long because of batch process, resulting in an elevated production cost, and this method can hardly be applied to a production process for massive products.
According to any of the conventional methods such as described above, care must be taken for correct removal of the desired thickness of film-forming resin in the step of exposing electroconductive particles, and it is very difficult to expose electroconductive particles by controlling the resin removal rate on both sides of the film. The object can hardly be accomplished unless at least the electroconductive particles are distributed in the same plane.
In the connection of fine electrodes in a functional test before mounting electronic parts such as LCD, TAB integrated circuits, bare chip integrated circuits, etc., there have been the problems such as inability of prior art to adapt itself to high fineness of electrodes, high cost, and poor electrical connection due to inability to absorb difference in height of electrodes. Specifically, in the connection mechanism using probes, since probes must be press-contacted with the respective electrodes, such probes are required to be fine in structure in correspondence to high fineness of the electrodes used, so that high-degree techniques are required for producing the probes. Also, it is not easy to arrange a plurality of probes in alignment with electrodes and this leads to an increased production cost. The method using an anisotropically electroconductive rubber sheet was also incapable of adapting itself to the mechanism involving high-fineness electrode and had the problems such as inability to retain insulation between adjoining electrodes and high connection resistance. In the case of an anisotropically electroconductive rubber sheet in which electroconductive particles are localized at the position corresponding to the electrodes, the electrode surface may be contaminated with organic matter, and in case an oxide layer is provided, it is hard to remove the surface insulating layer and the connection resistance is high.
In the case of the connecting system in which a conductive pattern formed on a flexible film is directly contacted with the electrode section of an electronic part to be tested, or in the case of the connecting mechanism in which an anisotropically electroconductive film having protuberant electrodes extending through an insulating polymer film is held between the part to be tested and a printed substrate for testing, since said conductive pattern and protuberant electrodes are made of a metal such as copper or gold and limited in amount of elastic deformation, variation in height of electrodes of the part to be tested must be compensated by bending deformation due to flexibility of the connected substrate and flexibility of the film substrate, and when this variation is large, the connecting performance lowers. In the ordinary printed wiring boards or integrated circuits, since solder resist or insulating protective film is present between the electrodes, the electrodes are in most cases positioned lower than said resist or protective film surface, so that the position of protuberant electrode is limited to the defined area.